K pre-intercalated manganese dioxide with enhanced Zn diffusion for high rate and durable aqueous zinc-ion batteries

Aqueous zinc-ion batteries (ZIBs) are attracting extensive research interest because of their safety and low toxicity. However, the development of ZIBs is hindered by sluggish reaction kinetics and serious structural degradation of the cathode materials. Here, tunnel-structured MnO 2 hierarchical nanotubes with high pre-intercalated K cation content (α-K 0.19 MnO 2 ) are proposed as a superior cathode for ZIBs. Specifically, the α-K 0.19 MnO 2 nanotubes are prepared via a self-sacrificial template method, including a neutral solvent hydrothermal intercalation and a subsequent annealing phase transformation process. When tested as cathodes for ZIBs, a subsequent H + and Zn 2+ intercalation mechanism at different voltage platforms is clarified. The water-solvated H + first inserts into tunnel cavities and the subsequent insertion of Zn 2+ into MnO 2 partially changes the MnO 2 phase from a tunnel-type structure to a layered-type structure (Zn-buserite). The high content pre-intercalated K cations in the layered-type matrix as pillars stabilize the layered structures and expand Zn 2+ migration channels, which can facilitate the diffusion of Zn 2+ in the MnO 2 cathodes. It is noteworthy that, a K-salt additive is employed to maintain the concentration of K + in the electrolyte with the aim of inhibiting the extraction of K + from the α-K 0.19 MnO 2 host material during cycling, thereby further boosting the cycling ability. α-K 0.19 MnO 2 nanotubes with high K + content as cathodes for zinc-ion batteries show high capacity, excellent rate capability and cycling stability.